1. Field of the Invention
The present invention relates to infrared objective systems with optical athermalization and achromatization.
2. Description of the Related Prior Art
The principle of passive athermalization has already been the subject of many publications and patents.
Thus, an optical configuration of the dyalite type with two convergent sets arranged to form an achromatic system has been described in patent application GBA-A-2 121 211, the optical configuration compensating for the variations in optical distance between rear lens and focal plane induced by the expansion of the support and the changes in the indices of the glasses with the temperature. It has an approximately achromatic assembly of two lenses and a rear assembly constituted by a doublet, a convergent lens made of As.sub.2 Se.sub.3 and a divergent lens made of germanium, which undergoes great variations in focal length variations with temperature.
There also exist known systems with different architectures, as described, for example, in patent application No. GB-A-2 161 616, the basic element of which is constituted by a bonded triplet, of which the first two lenses, one positively powered front lens made of zinc selenide or chalcogenide and one divergent lens made of zinc sulphide, provide achromatization while the rear divergent lens, made of germanium, enables athermalization to be achieved. However, in all prior art objective systems, the variations of the focal lengths and the sensitivities to thermal gradients are not adequately corrected.
There have also been proposed systems of objectives with mirrors. These systems are generally less sensitive to thermal fluctuations. However, such systems are rarely used because, firstly, they do not allow a sufficient field to be obtained and, secondly, they often contribute a central concealment, and thus entail mirror dimensions that are incompatible with the space factor constraints.
It is clear, from the foregoing, that dioptric configurations will therefore be preferred. However, most of the architectures proposed prove to be highly sensitive to thermal drifts when germanium is used for the manufacture of the lenses and when it is desired to make relatively long focal lengths for objectives.